Kefka without a doubt.
Distant second would be Chaos. Muhfucka fucks up the earth and then travels back in time to send the four fiends to the future to fuck up the world and to send him back in time so he can send them to the future so they can... OH FUCK.
"A portal to 2000 years in the past opens in the Temple of Fiends. There the Warriors discover that the four Fiends sent Garland (now the archdemon Chaos) back in time and he sent the Fiends to the future to do so, creating a time loop by which he could live forever. The Light Warriors defeat Chaos, thus ending the paradox, and return home. By ending the paradox, however, the Light Warriors have changed the future to one where their heroic deeds from their own time remain unknown outside of legend."
WIKIPEDIA FUCK YES.